1. Field of the Invention
The present invention relates generally to an organic electroluminescent light emitting device (which will hereinafter be often called an organic EL device for short) using an organic compound and its production process, and more particularly to an electron injecting electrode and its production process.
2. Description of the Background
In recent years, organic EL devices have been under intensive investigation. One such organic EL device basically includes a transparent electrode (a hole injecting electrode) of tin-doped indium oxide (ITO), etc. A thin film is formed on the transparent electrode by evaporating a hole transporting material such as triphenyldiamine (TPD). A light emitting layer of a fluorescent material such as an aluminum quinolinol complex (Alq.sup.3) is deposited on the hole transporting thin film. An electrode (an electron injecting electrode) is formed thereon from a metal having a low work function such as magnesium or Mg. This organic EL device attracts attentions because it can achieve a very high luminance ranging from several hundreds to tens of thousands cd/m.sup.2 with a voltage of approximately 10 volts.
An electron injecting electrode considered to be effective for such organic EL devices is made up of a material capable of injecting more electrons into the light emitting layer or electron injecting and transporting layer. In other words, the lower the work function of a material, the more suitable is the material as the electron injecting electrode. Various materials having a low work function are available. Regarding materials used as the electron injecting electrode of organic EL devices, for instance, JP-A 2-15595 discloses an electron injecting electrode material comprising plurality of metals other than an alkali metal, at least one of which has a work function of less than 4 eV, typically MgAg.
A preferable material having a low work function is an alkali metal. U.S. Pat. Nos. 3,173,050 and 3,382,394 disclose NaK as one example of the alkali metal. However, an electron injecting electrode made up of the alkali metal is inferior to that built up of MgAg, etc. in terms of safety and reliability, because the alkali metal has high activity and so is chemically unstable.
In efforts to increase the stability of electron injecting electrodes using alkali metals, for instance, JPA's 60-165771, 4-212287, 5-121172 and 5-159882 propose electron injecting electrodes using AlLi alloys. Reference is here made to the concentration of Li in the AlLi alloys disclosed in these publications and their production processes. (1) JP-A 60-165771 teaches that the concentration of Li is in the range of 3.6 to 99.8 at % (1 to 99 wt %) and preferably 29.5 to 79.1 at % (10 to 50 wt %), and the examples given therein show AlLi alloys having an Li content in the range of 15.8 to 79.1 at % (4.8 to 50 wt %). These AlLi alloys are all formed by an evaporation technique. (2) JP-A 4-212287 teaches that the concentration of Li is at least 6 at % and preferably 6 to 30 at %, and the example given therein shows an AlLi alloy having an Li content of 28 at %. Therein, these alloy films may be formed by resistance heating co-evaporation, electron beam evaporation or sputtering. However the example refers to an evaporation process alone. (3) JP-A 5-121172 discloses AlLi alloys containing Li at concentrations of 0.0377 to 0.38 at % (0.01 to 0.1:100 by weight), and the examples given therein show that AlLi alloy films containing Li at concentrations of 0.060 to 0.31 at % (0.016 to 0.08:100 by weight) are formed by resistance heating evaporation or electron beam evaporation. Also, the publication discloses that AlLi alloy films having Li contents of up to 15.9 at % (50 or lower:1000 by weight) are formed, and the examples given therein show that AlLi alloy films having Li contents of 29.5 to 61.8 at % (10 to 30 wt %) re formed. (4) JP-A 5-159882 discloses AlLi alloys having Li contents of 5 to 90 at % and the examples given therein show AlLi alloys having Li contents of 16 to 60 at %. Therein, these alloy films are formed by double-evaporation wherein resistance heating evaporation is applied to an Li source while electron beam evaporation is applied to the other.
However, the AlLi alloy electrodes set forth in publications (1), (3) and (4) are all formed by vacuum evaporation alone. Although publication (2) refers to the formation of AlLi alloy electrodes by sputtering, only vacuum evaporation is described in the examples therein. Thus, the examples gives nothing specific about sputtering.
When an evaporation process is used, an AlLi alloy is employed as an Li evaporation source because lithium is in itself inferior in terms of chemical stability, film-forming capability, and adhesion. Since these metals have varying vapor pressures, however, it is required to rely upon double evaporation (co-evaporation) with Al. A problem with double evaporation is, however, that it is not easy to gain composition control and so it is difficult to obtain the optimum mixing ratio in a stable manner for each batch. Thus, the actually obtainable Li concentration is shifted to a relatively high concentration side of 16 to 79 at %, and cannot be kept invariable. A high Li concentration is a factor in the degradation of devices because the composition is chemically unstable, resulting in deterioration of its film-forming capability and adhesion. In addition, devices having consistent quality cannot be obtained. When evaporation is carried out using a single evaporation source, on the other hand, the concentration of Li drops to 0.38 at % or lower, yielding an alloy having a high work function. This in turn gives rise to an electron injection efficiency drop, and so renders it difficult to obtain devices having practical enough properties.
An electron injecting electrode film formed by a vacuum evaporation process is poor in denseness, and adhesion to an organic layer interface, yielding an organic EL device with a light emission efficiency drop and dark spots produced by the delamination of the electrode. Thus, the obtained EL device offers problems in connection with its properties and service life, and the quality of what is displayed on it.
A material having a low work function, like Li, is usually formed thereon with an oxide layer, because the material is of high reactivity with respect to oxygen or moisture, and is usually handled in the atmosphere for feed, and supply purposes. To form an electron injecting electrode of high quality, it is desired to carry out evaporation after removal of the oxide layer form the surface of the material. However, this is difficult because, in a rare case alone, the oxide has a lower evaporation temperature or a higher vapor pressure than does a pure metal. It is thus not easy to form a high-quality electron injecting electrode consisting of a pure metal film. In addition, when a film obtained from such an oxide material by evaporation is formed on the interface between the electron injecting electrode and the organic layer or within the electrode, the resulting work function and electrical conductivity deviate from those of the pure metal. It is thus impossible to obtain any desired EL properties. From a practical perspective, the vacuum evaporation process have various productivity problems, among which it is required to make a material replacement or addition within a short time of period, a film having a large area is inconsistent in terms of composition control, film thickness and film quality, and consistent composition control, film quality reproducibility and consistent film quality are not obtained at an increased film forming rate.